10-04-2012, 11:51 AM
DRIVESHAFTS AND UNIVERSAL JOINTS
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The term "driveshaft" and "propeller shaft" are synonymous, since both describe the basic function that they are designed for, namely to transmit drive from the output shaft of the transmission to the rear axle assembly in order to drive, or propel, the vehicle forward.
Critical Speed
Every driveshaft has a critical speed. Critical speed is the point at which a rotating driveshaft begins to bow off its normal rotating centerline.
Drive shafts begin to vibrate as they approach critical speed. If they are operated at near critical speed for an extended period, they often fail. This can damage the vehicle and possibly injure persons nearby.
Operating angles.
Because any U-Joint operating through an angle has an inherent resistance to motion the allowable drive shaft operating angle is limited by the speed the driveshaft is expected to turn. The faster the speed the lower the angle must be.
Non-constant velocity
A shaft driven through an angle using a U-Joint experiences non-constant velocity, that is that the driven shaft speeds up and slows down twice during a revolution. If a shaft with one U-Joint working through an angle were connected to a drive axle the vehicle would speed up and slow down twice through every revolution.
Drive shaft inspection
To inspect the drive shaft for wear or damage, raise the car on a hoist. Look for undercoating or mud on the drive shaft. Check for missing balance weights, cracked welds, and other drive shaft problems. To check for worn U-joints, wiggle and rotate each U-joint back and forth. Watch the universal joint carefully. Try to detect any play between the cross and yoke. If the cross moves inside the yoke, the U-joint is worn and should be replaced.
Driveline Angularity
Driveline angularity is usually the last thing to look for when checking for vibration unless obvious changes have been made to the vehicle running height or length etc. but after eliminating other possible causes you may need to check the driveline angles.
To check the angles you will need a good protractor, most manufacturers print angularity worksheets that you can use to record the angles and almost all have computerized angularity programs that will do the calculating for you.
Lubrication
Some U-Joints are sealed and have no grease zerks all others must be lubricated regularly following the OEM manual recommendations, usually every 10,000 miles or 16,000 Kilometres. It is crucial that grease purges from all 4 bearing cups and it is a good idea to purge the cups until all of the original lube is gone to avoid incompatible greases from being mixed.